(1) Field of the Invention
The present invention relates to a water-in-oil emulsion explosive composition containing a specifically limited gas-retaining agent, and more particularly relates to a water-in-oil emulsion explosive composition having a high performance and an improved safety.
(2) Description of the Prior Art
There has been used since about 10 years a slurry explosive, which is one of water-gel explosives, from the view point of the safety in the production and handling.
Slurry explosive is less sensitive than dynamite, which had been used before the development of slurry explosive, and is required to contain bubbles in an amount larger that contained in dynamite in view of the keeping of detonation sensitivity.
Since several years, there has been sold in the market a water-in-oil emulsion explosive, which is one of water-gel explosives but is different from the slurry explosive in the structure, that is, has a structure wherein an aqueous solution of inorganic oxidizer salt is wrapped with a film of carbonaceous fuel.
The above described slurry explosive uses sensitizers, such as monomethylamine nitrate, ethylene glycol mononitrate, ethanolamine mononitrate, ethylenediamine mononitrate, aluminum powder and the like, as an essential component in view of the keeping of explosion performance. However, the water-in-oil emulsion explosive does not require to use such sensitizer. Therefore, the use of bubbles in the water-in-oil emulsion explosive has increasingly become more important than the use of bubbles in the slurry explosive.
As the bubbles, there can be generally used bubbles mechanially (physically) mixed or blown into an emulsion explosive, bubbles formed in an emulsion explosive by a chemical foaming agent, bubbles mixed into an emulsion explosive by a gas-retaining agent, such as hollow microspheres, and the like. Among them, the former two kinds of bubbles leak during the storage of the explosive for a long time to deteriorate the detonation sensitivity and other properties of the explosive during the storage, and are disadvantageous.
As the water-in-oil emulsion explosive containing hollow microspheres, there are known water-in-oil emulsion explosives, wherein glass hollow microspheres are used (U.S. Pat. Nos. 4,141,767, 4,149,916, 4,149,917 and 4,216,040), and water-in-oil emulsion explosives, wherein resin hollow microspheres are used (U.S. Pat. Nos. 3,773,573 and 4,110,134). In these water-in-oil emulsion explosives, hollow microspheres having an average particle size of about 80-120 .mu.m are generally used.
However, a water-in-oil emulsion explosive using hollow microspheres having the above described average particle size of about 80-120 .mu.m are lower in the detonation velocity than a water-in-oil emulsion explosive using hollow microspheres having an average particle size smaller than about 80 .mu.m and further have drawbacks that the explosive is high in bullet impact sensitivity, card gap sensitivity and the like, which are used as an indication of the safety of the explosive in its production.
While, in the glass hollow microspheres having a small average particle size, ones having a low bulk density (that is, ones having a relatively small shell thickness) result in a water-in-oil emulsion explosive composition having a poor resistance against dead pressing, and reversely ones having a high bulk density (that is, ones having a relatively large shell thickness) result in the explosive composition having a poor strength.
Further, resin hollow microspheres having a small average particle size are very poor in heat resistance, and therefore bubbles leak during the production of a water-in-oil emulsion explosive containing the resin hollow microspheres, resulting in an explosive having a poor explosion performance.
The inventors have made various investigations in order to overcome the drawbacks of water-in-oil emulsion explosive compositions containing conventional hollow microspheres, and found out that the use of specifically limited hollow microspheres results in a heat-resistant and safe water-in-oil emulsion explosive composition having improved explosion performances, such as detonation velocity, strength and the like, and have accomplished the present invention.